1. Field of the Invention
The present invention relates to a polarity controlling device. In addition, the present invention also relates to a cleaner and an image forming apparatus using the polarity controlling device.
2. Discussion of the Background
Recently, an increasing need exists for electrophotographic images with high image qualities (particularly, high resolution). Therefore, the particle diameter of toner, which is used for forming visual images in electrophotography, becomes smaller and smaller. On the other hand, a need exists for toner having low manufacturing costs and high transfer rate. In order to fulfill such a need, spherical toner prepared by a polymerization method has been used for electrophotographic image forming apparatus.
Electrophotographic image forming apparatus typically use a cleaner including a blade, which removes toner particles having a charge and remaining on the surface of an image bearing member (such as photoreceptors) even after a toner image on the photoreceptor is transferred. In such a blade cleaning method, a rubber blade is contacted with the surface of the photoreceptor. In this case, if the blade is not well contacted with the surface of the photoreceptor (i.e., if contact between the cleaning blade and the photoreceptor lacks precision), toner particles to be removed by the blade often pass through the nip between the blade and the photoreceptor, resulting in occurrence of a background development problem in that the background of a toner image formed on a receiving material sheet is soiled with such residual toner particles.
In this regard, when the cleaning blade is contacted with the photoreceptor at a high pressure in attempting to avoid such a background development problem, another problem which occurs is that the tip of the cleaning blade is turned in the opposite direction, and thereby a streak of toner particles, which are not removed by the blade, is formed on the surface of the photoreceptor, resulting in formation of an abnormal streak image.
Even when spherical toner is used, residual toner particles on a photoreceptor can be typically removed if the contact pressure of a cleaning blade is extremely high (specifically, not less than 100 gf/cm (i.e., 0.98 N/cm) in a linear pressure). However, in this case, a problem in that the lives of the photoreceptor and the cleaning blade shorten occurs.
In this regard, under normal conditions such that the contact pressure of a cleaning blade is 20 gf/cm (0.196 N/cm) and the diameter of the photoreceptor is 30 mm, the life of the photoreceptor is about 100 kp (1 kp=1,000 sheets of copy), and the life of the blade is about 120 kp. In contrast, when the contact pressure of the blade is 100 gf/cm, each of the lives of the photoreceptor and the blade is decreased to about 20 kp.
It is well known that spherical toner has good transfer properties but the cleaning property thereof is inferior to that of pulverization toner, which has irregular particle forms.
Instead of such blade cleaning methods, brush cleaning methods are used for removing toner particles. By using brush cleaning methods, abrasion of the surface of a photoreceptor can be reduced, and small and spherical toner particles can be well removed. An example of brush cleaning methods uses a brush contacted with the photoreceptor while rubbing the photoreceptor to collect residual toner particles on the photoreceptor, a toner collection roller contacted with the brush to collect the toner particles from the brush, and a blade (such as rubber blades) configured to remove the toner particles from the toner collection roller.
In this example brush cleaning method, a voltage is applied to the toner collection roller or both of the toner collection roller and the brush to perform cleaning using an electrostatic force. Therefore, the brush cleaning method is effective for removing spherical toner. However, in general, a voltage having a polarity opposite to the polarity of the toner used is applied in an image transfer process, in which a toner image on the photoreceptor is transferred to a receiving material, and therefore toner particles remaining on the photoreceptor after the image transfer process are a mixture of particles maintaining the original polarity, particles having the opposite polarity and particles having no polarity.
In attempting to remove such residual toner particles having a variety of polarities, a published unexamined Japanese patent application No. (hereinafter referred to as JP-A) 2005-265907 discloses a cleaning method in which residual toner particles are charged by a corona charger (i.e., a corotron charger) to control the polarity of the residual toner particles before the cleaning process, and the charged residual toner particles are then collected with two brushes, which are arranged side by side and to which positive and negative voltage are respectively applied. However, the cleaning device has to have two brushes and two toner collection devices, and thereby the size of the image forming apparatus is increased.
Recently, a need exists for miniaturized image forming apparatus. In order to fulfill the need, the diameter of the photoreceptor drum serving as an image bearing material becomes smaller and smaller. Therefore, the cleaning device used for the image forming apparatus has to be miniaturized. In attempting to fulfill the need, a relatively small cleaning device, in which a toner polarity controlling blade, to which a voltage is applied, is arranged to control the polarity of residual toner particles, and an electrostatic cleaning device is arranged on a downstream side from the blade to electrostatically collect the toner particles charged so as to have a positive or negative polarity, is proposed.
An example of the electrostatic cleaning device is that a brush roller and a collection roller are arranged while applying a voltage to the brush roller so that a potential difference is formed therebetween and thereby residual toner particles are adhered to the brush roller from the photoreceptor.
In such electrostatic cleaning methods, it is preferable that the charge distribution (i.e., q/d distribution) of the thus polarity-controlled residual toner particles falls in a certain range. In this regard, q represents the charge quantity of a toner particle and d represents the particle diameter of the toner particle. In this application, detailed explanation of charging of toner particles in an electrostatic cleaning device is omitted. However, charge injection to toner particles is basically caused although the quantity of the injected charge changes depending on the potential difference between the photoreceptor drum and the cleaning brush and the potential difference between the cleaning brush and the toner collection roller.
Therefore, the q/d distribution curve of the polarity-controlled toner particles is preferably present slightly apart from the point 0 fC/μm. Specifically, when the polarity of the charged residual toner particles is controlled to be negative, the lower end of the q/d distribution curve is preferably −0.2 fC/μm. In this case, the polarity of the toner particles is not changed (i.e., the negative polarity is maintained) even when the above-mentioned charge injection is caused.
The upper end of the q/d distribution curve is preferably −0.8 fC/μm. When the negative charge quantity of the charged residual toner particles increases, the attraction between the photoreceptor drum and the toner particles thereon increases, and therefore it becomes difficult to remove the toner particle from the photoreceptor. Therefore, the upper end of the q/d distribution curve is preferably −0.8 fC/μm. Thus, the q/d distribution curve of the charged residual toner particles preferably falls in a range of from −0.2 fC/μm to −0.8 fC/μm. In this case, the residual toner particles can be well removed from the photoreceptor.
However, conventional toner polarity controlling blades for controlling the polarity of toner have the following drawbacks (a)-(d).    (a) As illustrated in FIGS. 20A-20C (FIG. 20A illustrates the initial state), the width of the nip between a toner-polarity controlling blade 220 and a photoreceptor 100 changes as time elapses due to repetition of sticking and slipping of the blade at the nip. This is because such toner-polarity controlling blades typically have a relatively high friction coefficient. In this case, the amount of charge injected by the blade changes, and therefore the q/d distribution of toner particles (t) broadens to such a degree as not to fall in the targeted range of from −0.2 fC/μm to −0.8 fC/μm as illustrated in FIG. 21.    (b) As illustrated in FIG. 22A, one of the toner polarity controlling blades 220, which have been used for controlling the polarity of toner, is set on a setting table 221, and the tip edge of the blade is observed with a laser microscope 250 to determined the degree of abrasion of the tip edge. In FIG. 22A, character (a) denotes the field of view of the laser microscope 250. FIG. 22B is an enlarged view of the portion (a) in FIG. 22A. As illustrated in FIG. 22B, the tip edge portion of the blade 220 is abraded, wherein the edge in the initial state is illustrated by a dotted line. The profile of the tip edge portion is illustrated in FIG. 23. In FIG. 23, the tip edge in the initial state is also illustrated by a dotted line. Therefore, as illustrated in FIGS. 24A and 24B, the toner particles (t) pass through an abraded portion (b) of the blade 220. Accordingly, charges cannot be injected to the toner particles passing through the abraded portion (b), resulting in broadening of the q/d distribution to such a degree as not to fall in the targeted range of from −0.2 fC/μm to −0.8 fC/μm as illustrated in FIG. 25. In FIG. 25, the toner particles to which charges are not injected have a q/d distribution curve (sub-peak) around 0 fC/μm.    (c) As illustrated in FIG. 26A, some of toner particles having a polarity opposite to that of the voltage applied to the blade are attracted to the toner-exit-side of the blade. Since conventional toner polarity controlling blades typically have a poor toner releasability, the toner particles adhered to the blade are not removed therefrom even when the blade repeats sticking and slipping. In this regard, the blade having the “sticking” state is illustrated in FIG. 26B, and the blade having the “slipping” state is illustrated in FIG. 26C. In this case, the charge injection cannot be well performed by the blade, and therefore the q/d distribution curve broadens to such a degree as not to fall in the targeted range of from −0.2 fC/μm to −0.8 fC/μm as illustrated in FIG. 25.    (d) Urethane resins are typically used for conventional toner polarity controlling blades. In addition, in order to control the resistivity of the blades, electroconductive materials are included therein. Since electroconductive materials cannot be well dispersed in urethane resins, the resistivity of such conventional toner polarity controlling blades largely varies, resulting in broadening of the q/d distribution curve to such a degree as not to fall in the targeted range of from −0.2 fC/μm to −0.8 fC/μm as illustrated in FIG. 21.
Thus, the q/d distribution of residual toner particles, to which charges are injected by such conventional toner polarity controlling blades, tends to fall out of the targeted range of from −0.2 fC/μm to −0.8 fC/μm. Therefore, the residual toner particles cannot be well removed from the photoreceptor by a cleaning brush, which is located on the downstream side from the toner polarity controlling blade 220 (or 22). When materials in which electroconductive materials can be well dispersed are used for the toner polarity controlling blade, another problem in that the physical properties of the blade deteriorate, and thereby the blade cannot be practically used occurs.
JP-A 2004-272019 discloses a cleaning device using a blade having an edge, which is to be contacted with a photoreceptor and which has an angle greater than 90°. However, JP-A 2004-272019 does not disclose or suggest resin coating of such a blade having an edge having an angle greater than 90°.
Because of these reasons, a need exists for a toner polarity controlling device, which stably controls the polarity of residual toner particles so that the residual toner particles can be well removed from an image bearing member such as photoreceptors by an electrostatic cleaning method in order to prolong the life of the image bearing member and to produce high quality images.